Position determination

Once in flight, a principal task of the FMS is to determine the aircraft's position and the accuracy of that position.



 Simple FMS use a single sensor, generally GPS in order to determine position.

 But modern FMS use as many sensors as they can, such as VORs, in order to determine and validate their exact position.

 Some FMS use a Kalman filter to integrate the positions from the various sensors into a single position.


 Common sensors include:

  • Airline quality GPS receivers act as the primary sensor as they have the highest accuracy and integrity.

  • Radio aids designed for aircraft navigation act as the second highest quality sensors. These include;
    • Scanning DME (distance measuring equipment) that check the distances from five different DME stations simultaneously in order to determine one position every 10 seconds.
    • VORs (VHF omnidirectional radio range) that supply a bearing. With two VOR stations the aircraft position can be determined, but the accuracy is limited.
  • Inertial reference systems (IRS) use ring laser gyros and accelerometers in order to calculate the aircraft position. They are highly accurate and independent of outside sources. Airliners use the weighted average of three independent IRS to determine the “triple mixed IRS” position.

The FMS constantly crosschecks the various sensors and determines a single aircraft position and accuracy.


 The accuracy is described as the Actual Navigation Performance (ANP) a circle that the aircraft can be anywhere within measured as the diameter in nautical miles.

 Modern airspace has a set required navigation performance (RNP). 

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The aircraft must have its ANP less than its RNP in order to operate in certain high-level airspace.